Various processes or techniques have been used to manufacture optical fiber. Most of these processes include a step of manufacturing a glass preform and subsequently drawing the preform to create an optical fiber. A typical silica preform includes a core section and at least one cladding section that is concentric to the core section and has a lower refractive index than the core section. The difference in refractive index can be achieved, for example, by appropriate doping of the core and/or cladding. Most commonly, germanium oxide is used as a dopant in the core section to increase the refractive index of the core. Various other dopants that are known to increase the refractive index can be used instead of, or in combination with, the germanium dopant. Alternatively, or in addition, dopants that are known to reduce the refractive index, such as boron- and fluorine-containing dopants, can be added to the cladding.
Optical waveguide preforms are typically made using a chemical vapor deposition (CVD) technique. Examples include modified chemical vapor deposition (MCVD), vapor axial deposition (VAD) and outside vapor deposition (OVD). In all of these chemical vapor deposition processes, an intermediate product called a preform is prepared by deposited glass material (mostly silica) in the form of very small particles or soot onto a substrate. The preform has a structure similar to that of the final optical fiber, but also has notable differences, such as density, dimension and porosity. The final dimensions of the optical fiber are achieved by a drawing process in which the final diameter of the fiber is on the order of about 125 micrometers.
In the case of modified chemical vapor deposition, reactants flow through a reactor tube together with a mixture of carrying gases (e.g., argon and/or helium) and oxidizing gas (e.g., oxygen) in controlled quantities. The reaction tube is typically made of silica, and is heated by an external burner that may traverse the length of the tube as the tube is rotated. Silica and other glass components are deposited on the inside surface of the tube and reaction products exit the end of the tube opposite the end at which the reactants are added.
The outside vapor deposition process involves deposition of glass particles or soot on the outer surface of a cylindrical rod typically made of alumina or graphite. The rod is typically rotated and exposed to a flame of a burner. The reactants are injected into the flame together with a fuel gas, such as methane or hydrogen. The core material is deposited first, followed by the cladding material. When deposition is complete, the substrate rod (also known as a “bait rod”) is removed from the center of the porous preform and the preform is placed into a consolidation furnace. During consolidation, water vapor is removed from the preform. The high-temperature consolidation step sinters the preform into a solid, dense and transparent glass. The consolidated glass preform is then placed on a draw tower and drawn into one continuous strand of glass fiber.
The outside vapor deposition process has been employed for large scale production of optical fibers. Advantages include production of a purely synthetic optical fiber exhibiting enhanced reliability, the ability to manufacture optical fibers exhibiting precise geometrical and optical consistency, scalability, high production rates, and greater manufacturing independence (because there is no reliance on a separate supply of reactor tubes used in a MCVD process). However, a disadvantage with the OVD process is that the starting materials are not efficiently utilized. Accordingly, it would be desirable to provide a process for manufacturing an optical fiber having complex refractive index profile features in which there is a step change in the refractive index over a distance less than 2 micrometers in optical fiber space, which makes more efficient use of material.
Other techniques that have been explored for producing an optical fiber having complex refractive index profile features include modified chemical vapor deposition, which deposits soot of varying dopant concentrations in a single sintering step. The dopants in the MCVD process are delivered via flame hydrolysis through a burner.